1. Field of the Invention
The present invention relates to a projection aligner used in semiconductor device fabrication processes to project a mask pattern onto a substrate, a projection exposure method using the projection aligner, and a semiconductor device.
More specifically, the present invention relates to the structure and use of a projection aligner capable of reducing variations in the dimensions and the shapes of resist patterns caused due to variations in the thickness and physical properties of films formed on a substrate.
2. Background Art
A typical example to which the present invention is applied is the projection aligner used when fine circuit patterns are formed on a substrate in a photoengraving process. The present invention will be described below using a reduction projection aligner as an example.
FIG. 3 is a sectional view of a reduction projection aligner.
Referring to FIG. 3, reference numeral 1 denotes an exposure light source; 2 a fly-eye lens; 3 a "sgr" aperture; 3A a reflector; 3B a condenser lens; 4 a reticle; 5 a projection optical system; and 7 a wafer which is a substrate. Generally, exposure light emitted from the exposure light source 1 goes through the fly-eye lens 2, the "sgr" aperture 3, and the condenser lens 3B, illuminates the reticle 4, and reaches the wafer 7 after being condensed by the projection optical system 5, thereby forming a resist pattern on the surface of the wafer 7.
In recent years, with the high integration and miniaturization of semiconductor device circuit patterns, exposure devices used in the process of projecting and exposing a mask pattern onto a semiconductor substrate have employed higher resolutions by reducing the wavelengths of the exposure light and increasing the aperture number (NA) of the reduction projection lens. Currently, a projection aligner whose aperture number (NA) is no less than 0.7 is almost put into practical use, using ArF excimer laser light as an exposure light source.
However, the focal depth becomes shallow as the wavelength of the exposure light is reduced and the aperture number (NA) of the reduction projection lens is increased to enhance the resolution. As a result, the process variation tolerance is reduced.
Therefore, to faithfully and accurately form fine patterns on a substrate, the Chemical Mechanical Polishing technique, etc. is applied to enhance the flatness of the wafer. As the projection aligner, a scan-type projection aligner is adopted which performs an exposure while following the uneven surfaces of the substrate by use of data obtained through information sent from a pre-read focus sensor. Additionally, effort has been done with regard to the projection aligner to improve measurement accuracy thereof.
On the other hand, since the focal depth has recently become shallow and, as a result, the process variation tolerance has been reduced, as described above, variations in the dimensions and the shapes of resist patterns are affected by variations in the thickness of resist films and films formed on a substrate.
These variations will be specifically described with reference to FIGS. 4 through 6. FIGS. 4 through 6 are graphs showing the relationships among the thickness of an SiN film 7b, the thickness of a resist film 7c, and the reflectance of an wafer 7.
FIG. 4 shows a case in which the SiN film 7b is deposited on the Si substrate 7a to a thickness of 1000 xc3x85 and the KrF resist 7c is further coated thereon to a thickness of 5000 xc3x85. FIG. 4 indicates changes in the reflectance of the KrF excimer laser light from the wafer 7 with changes in each film thickness. In FIG. 4, the horizontal axis indicates the thickness of the SiN film 7b, while the vertical axis indicates the thickness of the resist film 7c. Changes in the reflectance of the wafer 7 are expressed by use of contour lines.
It is considered that the actual thickness of an SiN film varies by xc2x110%, while the thickness of a resist film varies by xc2x150 xc3x85. Accordingly, in FIG. 4, the thickness of the SiN film 7b changes from 900 xc3x85 to 1100 xc3x85, while the thickness of the resist film 7c changes from 4950 xc3x85 to 5050 xc3x85. As indicated in the figure, the reflectance of the wafer 7 considerably changes with changes in the thickness of the SiN film 7b and the resist film 7c. 
FIG. 5 is a graph showing changes in the reflectance of KrF excimer laser light from the wafer 7 with changes in the thickness of the SiN film 7b when the thickness of the resist film 7c is fixed. In FIG. 5, the horizontal axis indicates the thickness of the SiN film 7b, while the vertical axis indicates the reflectance of wafer 7. Assume that the actual thickness of an SiN film varies by xc2x110%. As can be seen from FIG. 5, the reflectance of the wafer 7 changes from 18% to 52%.
FIG. 6 is a graph showing changes in the reflectance of KrF excimer laser light from the wafer 7 with changes in the thickness of the resist film 7c when the thickness of the SiN film 7b is fixed. In FIG. 6, the horizontal axis indicates the thickness of the resist film 7c, while the vertical axis indicates the reflectance of the wafer 7. On a flat substrate, the thickness of a resist film can be controlled so that it is within a set value xc2x150 xc3x85. Accordingly, as can be seen from FIG. 6, the reflectance of the wafer 7 changes from 32% to 49%.
Incidentally, the reflectance of a substrate is inversely proportional to the amount of energy absorbed into the resist. That is, as the reflectance of the substrate becomes higher, a smaller amount of energy is absorbed into the resist. This means that in the case of the positive resist, the amount of sensed light decreases.
Specifically, as shown in FIG. 7, when the reflectance of the wafer 7 is higher than the reference reflectance for setting exposure amounts, the dimensions of formed resist patterns increase. When the reflectance of the wafer 7 is lower than the reference reflectance for setting exposure amounts, the dimensions of formed resist patterns decrease.
As described above, variations in the dimensions and the shapes of resist patterns are affected by variations in the thickness of the resist film and the films formed on a substrate. Of these variations, the variation in the thickness of the resist film can be reduced by flattening the wafer, which is done as a measure to cope with the shallowed focal depths. Accordingly, it is important to control the thickness and the physical properties of the film formed under the resist film on the substrate in order to reduce dimensional variations in resist patterns caused due to variations in the reflectance of the substrate.
However, since it is difficult to control variations in the thickness and the physical properties over the entire wafer surface, between wafers, or between lots, an antireflective coating is applied under the resist film and to the uppermost layer of the substrate in order to reduce influence of the reflection from the substrate. On the projection aligner side, on the other hand, such measures as adoption of a method for controlling exposure amounts, and reduction of uneven illumination may be applied.
However, to apply the antireflective film, it is necessary to add a step of antireflective film etching in the dry etching process after photolithography, which reduces dimensional controllability of the resist patterns. Furthermore, forming of the antireflective film increases the production cost as well as reducing the processing capability.
In order to solve the above problems, the present invention faithfully and accurately projects and exposes patterns which have been becoming highly dense and fine. Specifically, the present invention reduces variations in the dimensions and the shapes of resist patterns at low cost without lowering the processing capability and the dimensional controllability at the time of dry etching.
One aspect of the present invention is a projection aligner for projecting a mask pattern for fabrication of a semiconductor device onto a substrate to be processed. The projection aligner comprises a reflectance measuring mechanism for irradiating a substrate to be processed with exposure light and measuring a reflectance of the exposure light from the substrate to be processed, and a control mechanism for adjusting an intensity of the exposure light to a predetermined intensity by referring to the measured reflectance.
According another aspect of the present invention, in a projection exposure method, a substrate to be processed is irradiated with exposure light from an exposure light source. A reflectance of the exposure light from the substrate to be processed is measured. An appropriate intensity of exposure light for the substrate to be processed is determined based on the reflectance, and then a mask pattern is projected onto the substrate to be processed by irradiating with exposure light of the determined intensity.
Another aspect of the present invention is a semiconductor device fabricated by use of the projection exposure method according to the methods of the present invention.
Other and further objects, features and advantages of the invention will appear more fully from the following description.